The present invention relates to a method for controlling the injection and ignition in a direct-injection endothermic engine, in order to accelerate heating of the catalytic converter.
In vehicles which are provided with a direct-injection endothermic engine, i.e. which are provided with injectors which are disposed directly facing the combustion chambers, in order to inject the fuel into the latter, it is known to install a catalytic converter along the exhaust pipe, in order to suppress the pollutant substances which are present in the exhaust gases output by the engine.
Correct functioning of the catalytic converter is dependent on the fact that the temperature of the converter must reach a predetermined operating level, whereas the maximum efficiency of the converter, i.e. the capacity to suppress the pollutant substances in an optimum manner, is associated with the fact that the air/fuel ratio of the mixture supplied to the engine must be kept close to the stoichiometric value,. i.e. within a pre-determined interval which includes the stoichiometric value itself.
When the engine has been cold-started, the problem exists of heating the catalytic converter quickly up to the predetermined operating temperature, at the same time keeping the emissions of pollutants within the values pre-determined by the standards in force.
In order to solve this problem, it is known to use a method for controlling the injection and ignition, according to which, within the context of a single engine cycle, a rich mixture is supplied to a first number of cylinders, and a weak mixture is supplied to a second number of cylinders, such that in general the mixture which is supplied to the engine during the cycle is substantially stoichiometric. For each cylinder, the injection of fuel takes place before the compression stroke of the cylinder itself. The combustion in the cylinders supplied with rich mixture generates exhaust gases which are rich in carbon monoxide, whereas the combustion in the cylinders which are supplied with weak mixture generates exhaust gases which are rich in oxygen.
The carbon monoxide and the oxygen thus generated give rise to an exothermic reaction, which provides heat along the exhaust pipe, thus making it possible to accelerate the heating of the catalytic converter.
In addition, in the cylinders which are supplied with rich mixture, the moment of priming of the fuel is delayed relative to the nominal moment of priming, such that the increase of torque caused by the combustion of the rich mixture is partially compensated by a decrease in torque caused by the ignition delay. However, in the cylinders which are supplied with weak mixture, the ignition of the mixture takes place at the moment of nominal priming.
Although the above-described known method makes it possible to speed up the heating of the catalytic converter, and to keep the emissions within pre-determined limits, it has a disadvantage caused by the fact that the production of heat is obtained by means of operations (such as the enriching of the mixture, and the delay in the moment of ignition) which adversely affect the generation of the torque, relative to the torque value which is required in the actual operating condition of the engine. In fact, the compensation between the increase in torque which is caused by enriching of the mixture, and the decrease in torque which is caused by the delay in priming, is never perfect, with the consequence that the torque which is transmitted to the engine shaft, by the cylinders which are supplied with rich mixture, is different from the torque transmitted by the cylinders which are supplied with weak mixture. Thus, as far as the generation of torque is concerned, the various cylinders behave in a slightly different manner relative to one another, and this therefore affects slightly the driving control of the vehicle.
The object of the present, invention is thus to provide a method for controlling the injection and ignition in a direct-injection endothermic engine, which makes it possible to accelerate the heating of the converter, thus minimising the emissions of pollutants, and which, at the same time, overcomes the said disadvantage.
According to the present invention, a method is provided for controlling the injection and ignition of the air/fuel mixture in a direct-injection endothermic engine, in order to accelerate the heating of a catalytic converter which is disposed along an exhaust pipe of the engine itself, the method being characterised in that it comprises the steps of:
a) executing, during a single cycle of the engine, and for each cylinder of the engine itself, at least a first injection of fuel, during suction stroke and/or compression stroke of the cylinder itself, in order to supply a weak mixture;
b) priming combustion in each cylinder, at a priming moment which is pre-determined such that the combustion in each cylinder generates the same torque; and
c) executing, in at least one cylinder of the engine, a second injection of fuel, during the end of the expansion stroke and/or in the exhaust stroke of the cylinder itself, such that the ratio of air/fuel of the mixture as a whole which is supplied to the cylinder, is close to the stoichiometric value; the fuel which is injected in the said second injection giving rise to residual combustion in the presence of the oxygen-rich exhaust gases, which are derived from the combustion of the weak mixture; the residual combustion not affecting the torque generated, and producing heat in order to heat the catalytic converter.
By this means, the production of the heat which is necessary in order to speed up the heating of the catalytic converter is separate from the generation of the torque, since the ignition of the mixture supplied with the first injection generates torque, whereas the residual combustion of the fuel supplied in the second injection takes place when the exhaust valve which is associated with the cylinder is already open, and thus it does not affect adversely the torque transmitted to the engine. In addition, all the cylinders generate the same torque, and consequently the driving control of the vehicle is not affected.